The microstructure of a nuclear fuel pellet is determined in metallographic studies using techniques such as optical microscopy or electron microscopy.
For such studies, it is necessary to prepare the pellet by encapsulating it with a polymer such as an epoxy and/or phenolic polymer and/or polyester.
Once the studies have been carried out, the polymer, if some remains on the pellet, is irradiated by the ionizing radiation emitted by the nuclear fuel. It then degrades through a radiolysis reaction, whose products are in particular explosive gases, such as hydrogen, oxygen and methane.
For the purposes of shipping, storing and warehousing the pellet, it is therefore necessary to remove the polymer encapsulating all or part of the nuclear fuel pellet in order to suppress the risk of explosion, reduce the volume of waste that can be constituted by the encapsulated pellet, and avoid the polymer's neutron moderator effect in cases where the pellet is reused as a nuclear fuel.
Outside the nuclear field, different methods are available for the removal of a polymer. Thus, in the field of plastic processing, the polymer covering the matrix used for the injection molding of a part is removed using a thermal treatment in which the polymer is pyrolyzed in the presence of an oxidizing gas.
This pyrolysis generates volatile byproducts, which are easily removed through post-combustion in an auxiliary oven. However, it also generates solid residues, the destruction of which requires an additional step of air oxidation.
A pyrolysis treatment, when applied to a nuclear fuel pellet comprising uranium dioxide UO2, should allow as thorough a removal as possible of the polymer, but also of the solid residues that are generated by pyrolysis and which remain in contact with the pellet, since radiolyzing such residues may also generate explosive gases.
However, a treatment which combines pyrolysis and oxidation steps has the drawback of oxidizing UO2 into U3O8. This transformation is accompanied by a strong increase in the volume of the crystal lattice (by approximately 36), which the sintered pellet cannot accommodate. This leads to significant cracking and swelling of the pellet, thereby resulting in its fragmentation and the generation of U3O8 powder.
However, U3O8 in powder form has the drawbacks that it can be disseminated in the air and cause the total release of the fission gases that were initially contained in the nuclear fuel pellet (such as Krypton 85), with both phenomena increasing the risks of radiological contamination.